The present invention relates to image compression and decompression methods for bit-rate fixation and an apparatus therefor in image apparatuses, and more particularly to image compression and decompression methods and the apparatus therefor for effectively fixing the amount of bits by placing inter-screen differential distortions and periodically varying at a predetermined period the position of distortions when independently processing and encoding the respective pictures of the successive images with respect to a still image by a still image compression method. The present disclosure is based on the disclosure of Korean Patent Application No. 92-8993 filed May 27, 1992, which disclosure is incorporated herein by reference.
In the field of digital signal processing, i.e. in processing the image signal of a digital video cassette recorder (digital VCR), a high definition television (HDTV), digital video cameras, video phones or the like into the form of digital data, a technique has been used to compress the amount of image information. Especially, various compression encoding methods have been suggested, which enhance the recording efficiency of recording media used in digital VCRs.
The fixed amount of compressed digital data is, unlike that of the analog signal, not directly proportional to the fixed size of an original image. Therefore, the amount of compressed data has to be controlled so as to be equal for each field or a fixed-size frame, because fixing the amount of output bits is very important when performing trick-play such as a high speed playback.
In other words, to perform a special reproduction such as a high speed searching, screens of the compressed image data should have equal sizes, and the data on a particular section of the tape should correspond to the position where the screen of an image was originally positioned. However, complicated portions of an image screen consume a large quantity of bits in the encoding process, while simpler parts require fewer bits.
Accordingly, when it is necessary to encode and record the image signal corresponding to a screen or a segment within a track of a magnetic tape having a finite length in conventional digital VCRs, the amount of encoded bits exceeding one track results in a variable length encoding method for recording the excess encoded bits onto the next following track, or a fixed length encoding method for stopping the encoding of the remaining parts if the number of encoded bits reaches the preset number of bits. Until recently, conventional digital VCRs have employed one or the other of these two methods.
The aforementioned variable length encoding method encodes linearly quantized coefficients by allocating thereto a variable-length code. However, as the number of encoded bits is not fixed, unless the recording format of a recording medium is properly controlled, the bit streams would influence one another, so that the operations of editing and high speed reproduction would be apt to generate many errors.
On the other hand, the above fixed length encoding method encodes the quantization coefficients resulted from an adaptive quantization, by allocating thereto a fixed-length code. Here, the bit streams have no mutual influences on each other, which reduces error probability in reproduction. In this case, however, the limited amount of recorded information degrades the picture quality.
The data compression apparatus and the method thereof for maintaining equally the amount of compressed data by means of the variable length encoding method is disclosed in U.S. application Ser. No. 07/886,194 filed May 21, 1992 which has been applied for by the same applicant and which is incorporated herein by reference.
Actually, in the entropy encoding process of the above-described image compression apparatuses, errors between the expected consumption number of bits and the required number of bits are caused. In such cases, a certain part of the DCT coefficients is removed and the amount of bits is controlled to a fixed amount. However, discrete cosine transform (DCT) coefficients are compulsively eliminated so that the image quality may become unbalanced with those of neighboring blocks, which generates an inconsistent picture quality. In other words, in the process of eliminating the DCT coefficients, a tolerance distortion bound is set, and if the consumed amount of bits exceeds the tolerance distortion bound, the DCT coefficients would be removed in excess of this bound, which degrades the picture quality. Therefore, the coefficients should be removed within the tolerance distortion bound, to be entropy-encoded as such.
Accordingly, the DCT coefficients are removed within the tolerance distortion bound in the divided blocks of one picture screen. At this point, one screen (one frame or a predetermined number of fields) of the image should be compressed into an equal size so that the particular section's data on a tape may correspond to that of the original image screen. If more DCT coefficients are removed in latter sections of a screen as much as the exceeded amount of bits, the errors are getting larger between the expected amount of bits and those actually consumed in the latter half of a screen. Consequently, the distortion in the latter half of the screen is more severe than in the former half, which in turn degrades the picture quality.